Pulling rolls for use in manufacturing sheet glass

ABSTRACT

Pulling rolls ( 13 ) for glass ribbons ( 11 ) are provided. The rolls include a shaft ( 17 ), a plurality of heat-resistant discs ( 15 ) mounted on the shaft in face-to-face contact, and a pair of fittings ( 25  or  25,31 ) affixed to the shaft which apply an axial compressive force to the discs. The axial compressive force gives the portions ( 27 ) of the roll surface ( 33 ) which contact the glass ribbon during use a Shore D hardness at room temperature of between 30 and 55. The rolls achieve a long service life without applying excessive force to the glass ribbon or generating high levels of particulate contamination.

FIELD OF THE INVENTION

This invention relates to the manufacture of sheet glass. Moreparticularly, the invention relates to improved pulling rolls for use incontrolling the thickness of glass produced by, for example, theoverflow downdraw fusion process. The invention is of particular valuein the production of high quality sheet glass, which exhibits low levelsof surface defects. Such glass can be used as substrates for flat paneldisplays, e.g., liquid crystal displays.

BACKGROUND OF THE INVENTION

Pulling rolls are used in the manufacture of sheet glass to applytension to the ribbon of glass from which the sheets are formed and thuscontrol the nominal sheet thickness. For example, in the overflowdowndraw fusion process (see Dockerty, U.S. Pat. Nos. 3,338,696 and3,682,609), pulling rolls are placed downstream of the tip or root ofthe fusion pipe and are used to adjust the rate at which the formedribbon of glass leaves the pipe and thus determine the nominal thicknessof the finished sheet.

Pulling rolls are preferably designed to contact the glass ribbon at itsouter edges, specifically, in regions just inboard of the thickenedbeads which exist at the very edges of the ribbon. A preferredconstruction for such rolls employs discs of a heat resistant material,which are mounted on a driven shaft. Examples of this construction canbe found in Moore, U.S. Pat. No. 3,334,010, Asaumi et al., U.S. Pat. No.4,533,581, and Hart et al., U.S. Pat. No. 5,989,170.

A successful pulling roll needs to meet a number of conflictingcriteria. First, the roll needs to be able to withstand the hightemperatures associated with newly formed glass for substantial periodsof time. The longer a roll can last in such an environment the better,since roll replacement reduces the amount of finished glass a givenmachine can produce and thus increases the ultimate cost of the glass.

Second, the roll must be able to produce enough pulling force to controlthe glass' thickness. In order not to damage the central portion of theribbon that becomes the usable finished glass, the roll can only contactthe ribbon over a limited area at its edges. Thus, the required pullingforces must be generated using only this area. However, the forcesapplied to the glass cannot be too large since this can create surfacedamage which can propagate into the usable central portion of theribbon. Accordingly, the roll must achieve a balance between applyingtoo little and too much force to the edge regions of the glass.

Third, the pulling roll must not give off excessive amounts of particleswhich can adhere to the glass and form surface defects (known as“onclusions”). For glass that is to be used in such demandingapplications as substrates for flat panel displays, onclusions must bekept to very low levels since each onclusion will typically represent adefective region of the finished product (e.g., one or more defectivepixels). Because of the hot environment in which pulling rolls operate,finding materials which can apply sufficient pulling forces to a glassribbon and yet not give off particles when hot is a difficult challenge.

The existing pulling rolls have not been able to fully satisfy thesecompeting criteria of long life, controlled force application, and lowcontamination. The present invention addresses these shortcomings in theart and provides improved pulling rolls which achieve higher levels ofperformance than existing pulling rolls.

SUMMARY OF THE INVENTION

In accordance with a first aspect, the invention provides a roll (13)for pulling a glass ribbon (11) comprising:

-   -   (a) a shaft (17);    -   (b) a plurality of heat-resistant discs (15) mounted on the        shaft in face-to-face contact, said discs having circular outer        peripheries which together form a roll surface (33), one or more        portions (27) of which are adapted to contact the glass ribbon        during use of the roll, said discs comprising refractory ceramic        fibers (e.g., glass fibers), mica, and a heat resistant binder        (e.g., clay); and    -   (c) a pair of fittings (e.g., outer collars (25) or the        combination of an outer collar (25) and an inner collar (31))        affixed to the shaft, said fittings being spaced from one        another along the shaft by a distance which causes an axial        compressive force to be applied to the discs;    -   wherein when the discs are subject to said axial compressive        force, the one or more portions of the roll surface that are        adapted to contact the glass ribbon have a Shore D hardness at        room temperature (e.g., 23° C.) of between 30 and 55.

In accordance with a second aspect, the invention provides a method forproducing a pulling roll of the above type, said method comprising:

-   -   (a) providing a shaft (17), two fittings (e.g., outer collars 25        or the combination of an outer collar 25 and an inner collar        31), and a plurality of heat-resistant discs (15), said discs        comprising refractory ceramic fibers, mica, and a heat resistant        binder;    -   (b) assembling the plurality of heat-resistant discs on the        shaft and compressing the discs between the fittings; and    -   (c) monitoring the amount of force used in step (b) to compress        the discs and adjusting the number of discs if said force is        outside a predetermined range;    -   wherein the predetermined range in step (c) is selected so that        one or more portions (27) of the roll's surface (33) that        contact the glass ribbon (11) during use of the roll have a        Shore D hardness at room temperature of between 30 and 55. In        practice, the number of discs ultimately assembled on the shaft        and locked in place by the fittings will typically be in the        200-400 range depending on the size of the glass ribbon being        produced, although more or less discs can be used in the        practice of the invention if desired. For example, less discs        will typically be used in the embodiment of FIG. 6.

In accordance with each of the above aspects, the heat-resistant discsare preferably pre-fired prior to assembly to form the pulling roll sothat they exhibit substantially no compositional changes when exposed tothe temperatures at which the roll pulls the glass ribbon and thus willbe dimensionally stable under operating conditions. Also, prior to thepre-firing, the heat-resistant discs in addition to comprisingrefractory ceramic fibers, mica, and a heat resistant binder, preferablyalso comprise cellulose fibers, which are burned off during thepre-firing.

If desired, instead of employing pre-firing, any non-heat resistantmaterials which the discs contain (e.g., the cellulose fibers) can beremoved in situ as the roll is put into service. In such a case, thepressure used to assemble the roll should be adjusted (increased) totake account of the material that will burn off from the roll whenexposed to the hot environment surrounding the newly-formed glassribbon. In particular, the pressure should be increased so that afterthe in situ burnout, the one or more glass-engaging portions of the rollsurface will have the required Shore D hardness at room temperature ofbetween 30 and 55.

In practice, the pulling rolls of the invention have been found toachieve a long service life without applying excessive force to theglass ribbon or generating high levels of particulate contamination. Inpractice, the service life of a roll can often be prematurely shortenedby outside events that damage the roll (e.g., a fracture of the glassribbon). However, in the absence of such events, the pulling rolls ofthe invention easily achieve long service lives, e.g., service lives inexcess of 40 days, preferably in excess of 75 days, and most preferablyin excess of 100 days.

Additional features and advantages of the invention are set forth in thedetailed description which follows, and in part will be readily apparentto those skilled in the art from that description or recognized bypracticing the invention as described herein.

It is to be understood that both the foregoing general description andthe following detailed description are merely exemplary of theinvention, and are intended to provide an overview or framework forunderstanding the nature and character of the invention as claimed.

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate various embodimentsof the invention, and together with the description serve to explain theprinciples and operation of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing the use of pulling rollsconstructed in accordance with the invention to pull a ribbon of newlyformed glass.

FIG. 2 is an expanded view of the left hand portion of FIG. 1.

FIG. 3 is an end view of the pulling rolls and glass ribbon of FIG. 1.

FIG. 4 is a front cross-sectional view of a pulling roll constructed inaccordance with the invention.

FIG. 5 is a perspective view, partially in section, of a pulling rollconstructed in accordance with the invention and having a hexagonalshaft.

FIG. 6 is a front view of a pulling roll constructed in accordance withthe invention and having semi-compliant discs only in the vicinity ofthe places where the pulling roll makes contact with the glass ribbon.

The reference numbers used in the drawings correspond to the following:

-   -   11 glass ribbon    -   13 pulling roll    -   15 disc    -   17 shaft    -   19 bearing surface    -   21 retaining ring    -   23 groove    -   25 outer collar    -   27 glass-engaging portions of roll surface    -   29 non-glass-engaging portions of roll surface    -   31 inner collar    -   33 roll surface

DETAILED DESCRIPTION OF THE INVENTION

With reference now to the drawings, wherein like reference charactersdesignate like or corresponding parts throughout the several views,there is shown in FIG. 1 a perspective view of a pair of pulling rolls13 engaging a glass ribbon 11, e.g., a newly-formed ribbon of LCD glassformed by an overflow downdraw process. FIGS. 2 and 3 show theengagement of the pulling rolls with the glass ribbon in more detail.

As can be seen most clearly in FIG. 4, each pulling roll 13 includes ashaft 17 which carries a plurality of discs 15 which are held in placeby collars 25. Collars 25 are preferably locked in place on shaft 17 bysplit retaining rings (snap rings) 21 which seat in grooves 23 formed inthe shaft. Although retaining rings are preferred, other mechanismsknown in the art for attaching a collar to a shaft can be used ifdesired. Also, although collars are preferred, any type of fitting whichcan apply an axial compressive force to the heat-resistant discs whenaffixed to the shaft can be used in the practice of the invention. Itshould be noted that for each pair of fittings, only one of the fittingshas to be removable to allow discs 15 to be assembled onto shaft 17. Theother fitting can be permanently attached to the shaft or can beintegrally formed as part of the shaft. The word “affixed” as usedherein is intended to cover these various alternatives.

As shown in FIGS. 1-3, the fully assembled pulling rolls can includebearing surfaces 19 at the ends of shaft 17. Although various materialscan be used for the shaft and bearing surfaces, preferred materialsinclude a cast stainless steel alloy for the shaft, e.g., a centrifugalcast HP 45 alloy, in combination with stainless steel bearing surfaces,e.g., 330 stainless steel.

As illustrated by the right and left hand portions of FIG. 4, discs 15when first mounted on shaft 17 preferably all have, or are cut to have,the same diameter (see the right hand portion of FIG. 4) with an overallcontour subsequently being cut in the discs to leave raised areas(flats) 27 and recessed areas 29 (see the left hand portion of FIG. 4).As shown in, for example, FIG. 1, only raised areas 27 contact glassribbon 11 during use of the pulling roll. Although the pulling rollsshown in the figures have only two raised areas (two glass engagingportions), additional areas can be used if desired, e.g., four areas canbe used, with two areas being grouped at one end of the roll and theother two being grouped at the other end. Indeed, although notpreferred, the invention can be practiced with a roll having only oneglass-engaging portion.

FIG. 5 shows an alternate construction for pulling roll 13 where shaft17 has a non-circular cross section, specifically, a hexagonalcross-section in this figure. Such a non-circular cross-section can beused if excessive amounts of rotation of discs 15 about shaft 17 shouldbe observed in a particular application of the pulling roll of theinvention. In such a case, disc 15 will include a central aperture whoseshape corresponds to that of the non-circular shaft, i.e., the centralaperture instead of being round will have the same non-circular shape asthe shaft. A non-circular cross section can be formed in the shaft bystarting with a circular shaft and then machining the shaft to thedesired configuration, e.g., by machining a shaft composed of carbonsteel. Cast or drawn shafts can also be used in this embodiment of theinvention. For most applications, rotation of discs 15 about shaft 17does not occur to any substantial degree and thus shafts having acircular cross-section can be used and are preferred.

FIG. 6 shows a further alternate pulling roll configuration which can beused in the practice of the invention. As a general rule, reducing theouter diameter of the discs located in central recessed area 29 isadvantageous because in addition to avoiding contact with the glass,such a reduction minimizes at least to some extent the amount ofmaterial at the center of the roll which can become airborne and adhereto the glass ribbon to form defects. However, since the packing pressureon the discs in the glass-engaging portions of the rolls' surfacedecreases as the outside diameter of the center section is reduced, eventhough overall there is less material (less surface area), more of thematerial that is left has the potential to form contaminating particles.

Also, as a consequence of the reduction in packing pressure, the flatlocation becomes more prone to lose integrity and thus reduce the lifeof the roll. Typically, as the roll loses integrity, the flat locationis compacted and the discs at this location lose elasticity, whichmanifests itself through pronounced slippage of the glass ribbon. Inother cases, the integrity of the roll is so degraded that the flatsdeform enough to warp the glass ribbon so that the finished sheet doesnot conform to production specifications.

The embodiment of the invention shown in FIG. 6 addresses theseproblems. As shown in this figure, discs are completely removed from thecenter of the roll and are only used in the regions of the flats. Withless discs, the chance that particles from the discs will becomeairborne and generate defects on the glass ribbon is reduced. Also, withless discs, the time required to cut a specific roll profile is reduced,since there is now less material that needs to be removed to create thedesired pattern in the surface of the roll.

Because the surface of shaft 17 is completely exposed throughout thecenter section of the roll, a coating is preferably applied to the shaftto reduce the chances of oxidation of the metal making up the shaft,e.g., stainless steel. Suitable coatings include various ceramiccoatings, such as CERAK M-720 Black Ceramic Coating distributed by CetekLimited of Berea, Ohio.

To maintain the integrity of the discs, two inner collars 31 are affixedto shaft 17 using, for example, split retaining rings of the typediscussed above in connection with outer collars 25. These inner collarsenhance the packing pressure near the flats and therefore improve rolllife.

An additional benefit of this embodiment of the invention is that itallows a higher level of cooling of the glass at the location of thepulling rolls. Such additional cooling is of value since glasstemperatures at this location, which can be as high as 800° C., dictatethe amount of warp and stress the glass ribbon experiences and thus thefinished sheet will exhibit.

The embodiment of FIG. 6 can be used with a circular or non-circularshaft and can employ bearing surfaces 19.

Whatever configuration is used for the pulling rolls (i.e., theconfigurations shown in FIGS. 1-6 or other configurations now known orsubsequently developed), the ultimate success of a roll depends on theproperties of discs 15. In accordance with the preferred embodiments ofthe invention, discs of a specific composition which have been pre-firedand assembled in a particular manner are used to form the pulling rollsof the invention. Such discs have been found to meet the service life,controlled force application, and low contamination criteria discussedabove.

In general terms, the discs need to be substantially non-friable whensubjected to the forces and operating temperatures involved in pulling anewly-formed glass ribbon. In accordance with the invention, it has beenfound that discs comprising refractory ceramic fibers, mica, and a heatresistant binder have this property. In general terms, after anynon-heat resistant components have been burned off, the discs preferablycomprise by weight: 10-20% refractory ceramic fibers, 40-50% mica, and40-50% heat resistant binder.

More particularly, the discs are most preferably: (1) a composite ofclay, mica, glass fibers, and cellulose fibers; and (2) pre-fired sothat they exhibit substantially no compositional changes when exposed tothe temperatures associated with a pulling roll during use and will thusbe dimensionally stable under operating conditions. For example, thepre-firing can comprise heating the discs to a temperature of 700-800°C., preferably 760-800° C. (e.g., to 760° C.), holding them at thattemperature for a period of at least two hours, and then cooling themback to room temperature before assembly onto the pulling roll's shaft.

Pre-firing to such temperatures ensures that the disc materialexperiences all phase changes prior to use and thereby reducesvariability of the material after it has entered into service. Otherpre-firing times and temperatures can, of course, be used in thepractice of the invention so long as they provide a finished pullingroll whose composition is stable at the rolls' operating temperature.

A composite having the above composition is commercially available fromNichias Corporation of Tokyo, Japan, under the product designationSD-115. This product is supplied in sheets having a thickness of about 6millimeters and can be cut into discs having a circular outer perimeter(e.g., a diameter of 140 mm) and a center aperture (e.g., a circularaperture having a diameter of 64 mm) using, for example, a water jetcutter or an appropriately sized punch, punching being preferred.

Alternatively, sheets of the composite can be prepared by, for example:(1) forming a water slurry of clay (e.g., GLOMAX LL), mica, glass fibers(e.g., 10 wt. % Al₂O₃—SiO₂ fibers), wood fiber (e.g., HIBRITE fromCanada), and starch; (2) forming layers from the slurry using a rotarydrum “paper making” machine; (3) stacking the layers (e.g., 20 layers)to make a millboard 6 mm thick; (4) draping the millboard over a dryingrack to air dry for at least 24 hours; and (5) placing the drying rackin an oven at 110° C. for 24 hours.

Other commercially available composites besides the SD-115 material andother methods for producing such composites can, of course, be used inthe practice of the invention.

Even more important than disc composition and pre-firing, the assemblyof the discs onto the shaft of the pulling roll is critical to thesuccess of the invention. In particular, it has been found that thecompression force (pressing force) used in assembling the discs to formthe pulling roll must be carefully controlled to achieve a long servicelife. For example, as discussed in more detail below, it was found thata compression force of approximately 33,000 pounds resulted in a pullingroll having only a limited life (i.e., 39 days), while a compressionforce of 11,000 to 14,000 pounds resulted in a roll having a much longerlife (i.e., greater than 100 days). For discs having an outside diameterof about 5 inches (127 millimeters) and an inside diameter of about 2.5inches (63.5 millimeters), these forces correspond to axial pressures ofapproximately 2,250 psi for the high compressive force and approximately750-950 psi for the low compressive force.

Since compression force may vary with such factors as the compositionand number of discs used, a better measure of the amount of compressionwhich should be applied to the discs is the Shore hardness(specifically, the Shore D hardness) of the outer surface of theassembled roll and, in particular, the portions of the outer surfacethat will come into contact with the glass ribbon during use of theroll. In accordance with the invention, it has been found that the ShoreD hardness of these glass-engaging portions of the outer surface of thepulling roll must be in the range of 30 to 55 and preferably in therange from 40 to 55 to achieve a long service life. A Shore D hardnessin these ranges allows the surface of the pulling roll to comply withthe glass surface thereby reducing surface damage that results inchecking. The Shore D hardness is measured using commercially availableequipment.

Without intending to limit it in any manner, the present invention willbe more fully described by the following comparative examples.

COMPARATIVE EXAMPLE 1

This example compares various materials used to form the discs of apulling roll, as well as the compression force used to assemble thediscs on the pulling roll shaft. The following materials were tested:(1) ZYALITE—a mixture of aluminosilicate fibers and amorphous silica(colloid) sold by Vesuvius McDanel of Beaver Falls, Pa.; (2) ZYAROCK—aporous fused silica also sold by Vesuvius McDanel; (3) Nichias SD-115compressed at approximately 33,000 pounds (approximately 2,250 psi)(Dense Nichias); and (4) Nichias SD-115 compressed at 11,000 to 14,000pounds (approximately 750-950 psi) (Less Dense Nichias).

The results were as follows:

(1) ZYALITE—Rolls made from this material wore noticeably, with the wearparticles contaminating the virgin glass surface, i.e., the material wasfriable and formed onclusions. It is believed that these problems resultfrom low surface hardness in combination with anisotropy due to afibered microstructure.

(2) ZYAROCK—Rolls of this material caused a great deal of frictivedamage. This problem is believed to be due to the combination of thehigh penetration hardness and the high elastic modulus of the material.

(3) Dense Nichias—This material performed well compared to the abovematerials except roll life was short. The high density of the roll seemsto encourage damage to the glass ribbon by collecting glass on the rollsurface. Relatively high penetration hardness did not allow adheredglass to “sink” into the surface of the roll.

(4) Less Dense Nichias—This material performed well compared to all ofthe above. It delivered long service life without onclusions orsignificant wear of the material. Although not wishing to be bound byany particular theory of operation, it is believed that this superiorperformance was due to the material having a low elastic modulus and lowpenetration hardness. The material was able to conform to the glasswithout generating excessive stress and glass particles on the rollsurface were able to become “embedded” in the material without causingexcessive damage.

COMPARATIVE EXAMPLE 2

This example illustrates the importance of controlling the compressionforce applied to the discs in the assembly of the pulling roll. Fiverolls made from the Nichias SD-115 material were tested. The rolls hadthe following properties and service lives where the reference in thistable to compression of the discs with high pressure means a pressingforce of approximately 33,000 pounds (approximately 2,250 psi) andcompression with low pressure means a pressing force of 11,000 to 14,000pounds (approximately 750-950 psi):

Roll Service Number Characteristics Life 1 Discs fired to 640° C.; hexshaft of carbon 3 days steel; discs compressed on shaft with highpressure. 2 Discs fired to 640° C.; circular shaft of cast 22 days HP 45alloy; discs compressed on shaft with high pressure. 3 Discs fired to760° C.; circular shaft of cast 39 days HP 45 alloy; discs compressed onshaft with high pressure. 4 Unfired discs; hex shaft of carbon steel; 77days discs compressed on shaft with high pressure. 5 Discs fired to 760°C.; circular shaft of cast >100 days HP 45 alloy; discs compressed onshaft with low pressure.

The data in this table assumes that the life of the roll was notprematurely shortened by extraneous process damage. The enhanced servicelife of roll 5 is evident from this table thus illustrating theimportance of the pressing force used during assembly to the ultimateusefulness of the pulling roll.

Roll 4 is believed to have had a longer service life than rolls 1-3because although this roll was pressed with high pressure, it was notpre-fired and as a result, it experienced in situ burnout. The in situburnout and concomitant loss of non-heat resistant material produced areduction in the effective pressing force. Accordingly, roll 4 behavedas if it had undergone pre-firing and had then been pressed with a lowpressing force.

Rolls 2 and 3 are believed to have had longer service lives than roll 1because these rolls used stainless steel, rather than carbon steel,shafts. The stainless steel shafts had higher CTE's than the carbonsteel shaft and thus produced a greater reduction in the effectivepressing force on the discs as the roll was heated to operatingtemperature. That is, in both cases, the shaft expanded more than thediscs, but in the case of the stainless steel shafts, more expansionoccurred and thus the discs were under less compression during use,which enhanced their service life.

Although specific embodiments of the invention have been described andillustrated, it will be apparent to those skilled in the art thatmodifications and variations can be made without departing from theinvention's spirit and scope. The following claims are thus intended tocover the specific embodiments set forth herein as well as suchmodifications, variations, and equivalents.

1. A roll for pulling a glass ribbon comprising: (a) a shaft having afirst end and a second end; (b) a plurality of heat-resistant discsmounted on the shaft in face-to-face contact, said discs having circularouter peripheries which together form a roll surface, one or moreportions of which are adapted to contact the glass ribbon during use ofthe roll, said discs comprising refractory ceramic fibers, mica, and aheat resistant binder; and (c) a pair of fittings affixed to the shaft,said fittings being spaced from one another along the shaft by adistance which causes an axial compressive force to be applied to thediscs; wherein when the discs are subject to said axial compressiveforce, the one or more portions of the roll surface that are adapted tocontact the glass ribbon have a Shore D hardness at room temperature ofbetween 30 and
 55. 2. The roll of claim 1 wherein: (a) the shaftcomprises a pair of grooves, (b) the pair of fittings comprise a pair ofcollars and a pair of retaining rings, and (c) the pair of collars isaffixed to the shaft through engagement of the retaining rings with thegrooves and collars, one retaining ring engaging one of the grooves andone of the collars and the other retaining ring engaging the othergroove and the other collar.
 3. The roll of claim 1 wherein: (a) theplurality of heat-resistant discs comprises a first plurality of discsin the region of first end of the shaft and a second plurality of discsin the region of the second end of the shaft, said first and secondpluralities of discs each having an inboard most disc; and (b) the rollcomprises two additional fittings, one of which engages the inboard mostdisc of the first plurality of discs and the other of which engages theinboard most disc of the second plurality of discs.
 4. The roll of claim3 wherein the portion of the shaft between the additional fittings isfree of discs.
 5. The roll of claim 4 wherein the portion of the shaftbetween the additional fittings is coated with a heat resistantmaterial.
 6. The roll of claim 5 wherein the heat resistant material isa ceramic coating.
 7. The roll of claim 1 wherein the shaft has acircular cross section.
 8. The roll of claim 1 wherein the shaft has anon-circular cross section.
 9. The roll of claim 1 wherein the shaft isa cast stainless steel alloy.
 10. The roll of claim 1 wherein theheat-resistant discs are pre-fired so that they exhibit substantially nocompositional changes when exposed to the temperatures at which the rollpulls the glass ribbon.
 11. The roll of claim 10 wherein theheat-resistant discs are pre-fired at a temperature of at least 700° C.for a period of at least two hours.
 12. The roll of claim 10 wherein theheat-resistant discs prior to pre-firing comprise cellulose fibers whichare burned off during the pre-firing.
 13. The roll of claim 1 wherein inthe absence of externally caused damage, the roll has a service life ofat least 40 days.